1. Field of the Invention
The present invention relates to a toner for developing an electrostatic image, a developer, an image forming apparatus, a process for forming an image, a process cartridge, and a process for measuring porosity of the toner.
2. Description of the Related Art
In an electrophotographic device, an electrostatic recording device or the like, an electrostatic latent image is formed on a photoconductor, to which a toner is attracted. The toner is transferred to a transfer material, such as a sheet of paper, and then fused to the transfer material by heat and thus a toner image is formed. To form a full-color image, it is generally done by using four toners of different colors consisting of black, yellow, magenta, and cyan. Development is carried out for each color, each layer of toner is overlaid on the transfer material to form the toner image, and the toner image is then heated and simultaneously fused to obtain a full-color image.
In general, for a user who is accustomed to commercial prints such as offset lithographic prints, images created by full-color copiers are still not at a satisfactory level, and demands are high for further improving the quality to achieve the fineness and resolution that are comparable to those of photographic and offset prints. It is known that in order to improve the quality of an electrophotographic image, the diameters of toner particles are preferred to be small and the distribution of particle diameter is preferred to be narrow.
A latent image, either electric or magnetic, is made visible by toner. Toners used for developing an electrostatic image generally include colored particles comprising a colorant, a charge control agent, and other additives all in a binder resin. Processes for manufacturing toner can be categorized broadly into pulverization (grinding) and polymerization. Pulverization is a process in which a colorant, a charge control agent, an offset preventing agent, and the like are melted, mixed, and evenly dispersed in a thermoplastic resin, after which obtained composition is crushed into small particles and classified to obtain the toner. With pulverization, toners having somewhat favorable properties can be manufactured, but materials that can be used for the toners are limited. For instance, the composition made by melting and mixing is to be crushed and classified using an apparatus that is economically affordable. For this requirement, the composition after the melting and mixing is preferred to be sufficiently brittle. Therefore, when the composition is actually crushed into particles, the distribution of particle diameters tends to be wide spread. The drawback is that the yield may become extremely low when one tries to obtain a reproduced image with favorable resolution and tone because a portion of the toner particles, for example, minute particulates of 5 μm or less in diameter and large grains of 20 μm or more, is to be removed by classification. In addition, it is difficult in pulverization to evenly disperse a colorant, a charge control agent, and the like within a thermoplastic resin. Uneven dispersion of the agents and additive may adversely affect the flowability, developability, durability, image quality, and the like of the toner.
To overcome such problems in pulverization, toner particles are recently made by other processes such as suspension polymerization (refer to Japanese Patent Application Laid-Open (JP-A) No. 09-43909). However, toner particles manufactured by suspension polymerization have a drawback of poor cleanability although they are spherical. For development and transfer of low toner coverage image, there is little residual toner that is not transferred and therefore there is no concern of insufficient cleaning of toner. However, when the toner coverage of an image is high, e.g. a photographic image, a paper jam or the like may result in building up of non-transferred residual toner on a photoconductor on which toner is forming an image but not transferred. Accumulation of such residual toner leads to background shading. Moreover, residual toner contaminates components such as a charging roller, which charges a photoconductor by contact charging, and subsequently reduces the charging performance of the charging roller. Furthermore, concerns for toner particles formed by suspension polymerization include unsatisfactory fusibility at low temperatures and a large amount of energy required for fusion.
On the other hand, another process for manufacturing toner particles is disclosed in Japanese Patent (JP-B) No. 2537503 in which emulsion polymerization is used to form resin particulates, which are subsequently associated to obtain toner particles having irregular shapes. However, toner particles formed by emulsion polymerization have a large amount of residual surfactants inside the particles as well as on the surface thereof, even after being washed by water, which reduces the environmental stability of toner charge, increases the distribution of the amount of charge, and causes background shading on a printed image. In addition, the residual surfactant contaminates photoconductor, charging roller, developing roller, and other components, thus causing problems such as insufficient charging performance.
On the other hand, for the fusing process by contact heating, in which heating members such as a heating roller are used, the toner particles must possess releasability (which may be referred to as “offset resistance” hereinafter) from the heating members. In such case, the offset resistance can be improved by allowing a release agent to exist on the surface of the toner particles. In contrast, methods to improve offset resistance are disclosed in JP-A No. 2000-292973 and JP-A No. 2000-292978 in which resin particulates are not only contained in toner particles, but are concentrated at the surface of the toner particles. However, this approach brings up an issue in which the method increases the lower limit fusing temperature at which toner is fused and therefore is unsatisfactory in low temperature fusibility, i.e. energy-saving fusion.
In addition, this process, in which resin particulates obtained by emulsion polymerization are associated to provide irregular-shaped toner particles, has another problem. Generally, release agent particulates are additionally associated to improve the offset resistance. However, the release agent particulates are captured inside the toner particles and therefore the improvement of the offset resistance is not sufficient. Moreover, since each toner particle is formed by a random adhesion of resin particulates, release agent particulates, colorant particulates, and the like which are molten, the composition (the ratio at which each component is contained), molecular weight of the resin, and the like may be varied among obtained toner particles. As a result, the surface properties of toner particles are different from one another, and it is impossible to form stable images for a long period. Additionally, in a low-temperature fusing system, the resin particulates that are concentrated at the surface of the toner particles inhibit fusing and therefore the range of fusing temperature is not sufficient.
Recently, a new manufacturing process called emulsion-aggregation (EA) has been suggested (refer to JP-B No. 3141783). In this process, particles are formed from polymers that are dissolved in an organic solvent or the like whereas in suspension polymerization, particles are formed from monomers, and the emulsion-aggregation is said to be advantageous in that, for example, there is a larger selection of resins that can be used and polarity can be controlled. Furthermore, the emulsion-aggregation is said to be advantageous in that it is possible to control the structure of toner particles (core/shell structure control). However, the shell structure is a layer consisting only of a resin and the purpose thereof is to reduce the exposure of pigment and wax to the surface. The purpose is not to alter the structure in the resin, and the structure is not capable of achieving such purpose (from The 4th Joint Symposium of The Imaging Society of Japan and The Institute of Electrostatics Japan, 2002 Jul. 29). Therefore, although the toner particle has a shell structure, the surface of the toner particle is a usual resin without any ingenious feature so that when the toner particle is targeted at fusing at a lower temperature, it is not satisfactory from the standpoint of anti-heat preservability and environmental charge stability and this is a concern.
In any of the above-mentioned processes, that is, the suspension polymerization, the emulsion polymerization, and the emulsion-aggregation, styrene-acrylic resins are generally used. Polyester resins are difficult to be made into particles, and it is uneasy to control particle diameter, diameter distribution, and particle shape. Also, their fusibility is limited when the aim is for fusing at a lower temperature.
On the other hand, it is known that polyester modified by urea bonds is used for anti-heat preservability and low-temperature fusing (refer to JP-A No. 11-133667). However, the surface is not specially ingenious, and the environmental charge stability is not satisfactory especially when the conditions are harsh.
Much work has been done from various angles of approach in the field of electrophotography to improve image quality, and it is being more and more recognized that reducing the size and increasing the sphericity of the toner particle are extremely effective. However, as the diameter of toner particles becomes smaller, the transferability and fusibility tend to decrease, and image quality becomes poor. On the other hand, it is known that by making toner particles spherical, the transferability is improved (refer to JP-A No. 09-258474). In such situation, ever-faster image formation is desired in the field of color copiers and color printers. For the faster printing, the “tandem method” is effective (as disclosed in JP-A No. 05-341617). The “tandem method” is a method in which images formed by respective image forming units are sequentially transferred and overlaid on a single sheet of paper that is advanced by a transfer belt, so that a full-color image is obtained on the sheet. A color image forming apparatus using the tandem method is characterized in that various kinds of paper can be used, the quality of full-color images is high, and full-color images can be formed at high speed. The high-speed output of full-color images is especially characteristic and no other color image forming apparatuses have that characteristic. On the other hand, there are other attempts to increase the speed, while improving the image quality, by using spherical toner particles. However, while toner particles are needed to have quick fusibility in order to accommodate to high-speed output, no spherical toner particle with a good fusibility as well as low-temperature fusibility has been realized to date.
In addition, after the manufacture of a toner, environments during storage and transport, such as hot and humid, or low and dry, are severe for the toner. There are demands for a toner having an excellent preservability where toner particles do not coagulate even after being stored in such environments and degradation in chargeability, flowability, transferability, and fusibility is none or very little. However, especially for spherical toner particles, no effective way that is capable of overcoming such issues has been found to date.
Each of the conventional pulverized toner and the chemical (polymerized) toner is fully packed with toner composition, bringing about its performance ordinarily. However, it is difficult to satisfy the following two at the same time: i) prevention of degradation in developability and transferability such as charge stability, which degradation is involved in making the toner particle smaller, and ii) effect of smaller toner particle that the amount of the toner's adhesion to paper and the like per unit area is reduced (low M/A [mass area]), which effect has a significant weight. Therefore, such a method is preferred as to reduce the amount of the toner's adhesion (low M/A [mass area]) while keeping the toner particle diameter as small as possible and securing the developability, transferability, fusibility and the like.